1. Field of the Invention
The present invention relates to a carrier head for chemical mechanical polishing, and more particularly to a carrier head for chemical mechanical polishing that can improve polishing uniformity.
2. Description of the Related Art
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. Therefore, there is a need to periodically planarize the deposited layer surface to provide a flat surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. CMP is a fairly complex process, and it differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing of the substrate.
An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the CMP apparatus.
A recurring problem in CMP is non-uniformity of the polishing rate across the surface of the substrate. One source of this non-uniformity is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency for the substrate edge to be polished at a different rate than the center of the substrate. Another source of non-uniformity is termed the xe2x80x9ccenter slow effectxe2x80x9d, which is the tendency of center of the substrate to be underpolished. Yet another source of non-uniformity is termed as the fast band effect. The fast band effect causes an annular region of the substrate, the center of which is located approximately 15 millimeters to 20 millimeters from the substrate edge, to be significantly over-polished. This annular region may be about 20 millimeters wide. These non-uniform polishing effects reduce the overall flatness of the substrate and the substrate area suitable for integrated circuit fabrication, thus decreasing the process yield.
FIG. 1 shows a conventional carrier head for CMP. Carrier head 100 comprising a main frame 102, a retaining ring 104, an edge load ring 106, a support plate 108 having a incision or lip 110, and a flexible membrane 112 are shown. Generally, carrier head 100 holds a substrate such as a wafer in position against a polishing pad and distributes a force across the back surface of the substrate. Support plate 108 as well as incision 110 are composed of a rigid material, such as a stainless steel. During polishing, a bladder (not shown) is used to cause incision 110 of support plate 108 to press the edge of flexible membrane 112 against the wafer being polished, thereby creating a tight contact between the wafer and flexible membrane 112. However, the rigidity of incision 110 and the winding phenomena of flexible membrane 112 during polishing would present non-uniformity of polishing such as the fast band effect and reduce the overall flatness of the wafer and the wafer area suitable for integrated circuit fabrication, thus decreasing the process yield.
Accordingly, it would be necessary to provide a CMP apparatus which ameliorates some, if not all, of these problems. The advantages of this invention are that it solves the problems mentioned above.
It is therefore an object of the invention to provide a modified carrier head which can effectively improve polishing uniformity.
It is another object of this invention to provide a carrier head which can overcome the fast band effect.
It is a further object of this invention to provide a carrier head which can provide a overall flatness of the substrate being polished and an improved process yield.
In one embodiment of this invention, the invention provides a carrier head for chemical mechanical polishing, said carrier head comprising: a main frame; a support plate disposed in said main frame having a non-rigid incision ring surrounding said support plate, said incision ring having a downwardly-projecting non-rigid incision; and a flexible membrane disposed under said support plate extending around the edges of said support plate, wherein said flexible membrane applies a load from said main frame on a substrate being polished against a polish pad.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.